A typical electronic image sensor contains an array of pixels. Each pixel contains a light sensitive region called a photodetector and an element to transfer or read out the charge collected in the photodetector. A photodetector is a simple PN or NP diode. An electron sensing photodetector generally includes a single n-type implant disposed inside a p-type substrate or a p-type well. Alternatively, a hole sensing photodetector has a p-type implant formed inside an n-type substrate or n-type well. In addition to the diode implant, a shallow pinning implant having an opposite conductivity type to that of the photodetector is placed at the substrate surface. The pinning layer pins the surface potential and reduces dark current generation from the surface states.
FIG. 1 is a cross-sectional view of a first image sensor in accordance with the prior art. Image sensor 100 includes diode implant 102 and pinning layer 104, which together form photodetectors 106. Isolation regions 108 are disposed between photodetectors 106 to electrically isolate each photodetector from adjacent photodetectors. Photo-generated charge carriers generated in the depletion depth 110 are collected and stored by the diode implant 102. Photo-generated charge carriers generated below depletion depth 110 under any given photodetector can diffuse to an adjacent photodetector, an undesirable effect known as electrical crosstalk.
One technique for reducing electrical cross talk is to push the depletion depth 110 deeper into substrate 112. This can be accomplished by adding a cascading or chain of light implants to the main diode implant. This technique is disclosed in United States Patent Application Publication US2007/0069260.
To effectively isolate the deep chain photodetector, the isolation regions also need a chain implant to form deeper isolation regions. This structure is shown in FIG. 2. Image sensor 200 includes diode implants 202a, 202b, 202c, 202d and isolation implants 204a, 204b, 204c, 204d formed in substrate 206. Collectively diode implants 202a, 202b, 202c, 202d form deep photodetectors 208 and isolation implants 204a, 204b, 204c, 204d form deep isolation regions 210.
FIGS. 3-5 are cross-sectional views of a method for forming the photodetectors and isolation regions shown in FIG. 2. Substrate 206 in FIG. 3 has been processed to the point where the next fabrication step is to perform the isolation region implants. Substrate 206 has a screening pad oxide 300 on the surface. A masking layer 302 is deposited over substrate 206 and patterned to provide openings 304 that expose the surface of pad oxide 300. The remaining masking layer 302 is positioned over the portions of the image sensor that will not be implanted. So the remaining masking layer 302 has a thickness that is sufficiently thick enough to block the implant, for example 2-3 microns.
Next, as shown in FIG. 4, a series of implants (represented by arrows) is performed to implant dopants into substrate 206 and form isolation implants 204a, 204b, 204c, 204d. Each implant is performed with a different energy so the dopants will reside at different depths within substrate 206. Additional fabrication steps are performed after the formation of the isolation implants, such as the formation of gates, but these steps are omitted from this description for the sake of simplicity and ease of understanding.
Masking layer 302 is then removed and a new masking layer 500 deposited over substrate 206 and patterned to form openings 502 (see FIG. 5). A series of implants (represented by arrows) is performed to implant dopants into substrate 206 and form diode implants 202a, 202b, 202c, 202d. Each implant is performed with a different energy so the dopants will reside at different depths within substrate 206. Masking layer 500 is then removed and the structure shown in FIG. 2 is obtained.
One design goal is to maximize the size of the photodetector region to maximize the pixel sensitivity and charge handling capability. Based on this, the size of the isolation regions is kept to a minimum. An isolation width between 0.3 to 0.4 microns is not uncommon for a pixel size as small as 1.4 microns. But patterning openings 304 (FIG. 3) to such a minimum feature size in a resist having a 2-4 micron thickness is difficult if not impossible. The isolation region dopants are usually implanted into the substrate at a 7 degree tilt. But with a minimum feature size of 0.3 to 0.4 microns, and with the high height to width aspect ratio, the implant angle needs to have a 0 degree tilt to allow the implant to be implanted into substrate 206. Otherwise, the implant will be masked by the shadowing effect, which can lead to implant channeling variation across substrate 206. Additionally, the resist profile tends to have an 80 degree side slope to the side. Since the masking layer is very thick, the openings at the top are much wider than the openings at the bottom of the resist 302 to obtain the desired minimum feature size.